1. Field of the Invention
The present invention relates to an end face polishing method for polishing an end face of a workpiece in the form of, for example, a rod-shaped member such as a fiber for optical communication or a ferrule in which the fiber for optical communication is held.
2. Description of the Related Art
The fiber for optical communication is used by polishing simultaneously and smoothly the end face of the ferrule and the end face of the fiber to be a mirror surface after the fiber is fixedly adhered to an inside of a center hole of the ferrule which is a main member of a connector. When polished surfaces of the polishing finished ferrule and fiber are not perpendicular to a central axis of the ferrule or there is a flaw on the polished surface, accuracy of facing position is deteriorated and loss is increased in the optical connector in which the ferrules are oppositely connected each other. Therefore, the polishing finish of the high accuracy is required for the polished surface of the ferrule including the optical fiber.
For example, in JA-A-3-26456, there is disclosed an optical fiber end face polishing apparatus of the related art for polishing the end face of the ferrule including the optical fiber. The optical fiber end face polishing apparatus disclosed in the publication has an eccentric board which rotates on a concentric circle of a rotation disk and a planetary gear which transmits a rotation of a motor for a revolution to the eccentric board, connects them with a polishing board to rotate and revolve the polishing board, and polishes the end faces of a number of ferrules which are held to a jig board by a support portion provided in a main body of the apparatus while the end faces of the ferrules are pressed to an abrasive member fixed to the polishing board.
In case that the end face of the ferrule is polished by using the above-described optical fiber end face polishing apparatus, generally, there is employed an optical fiber end face polishing method in which, in order, a rough polishing process is carried out by using the abrasive member having given hardness, an intermediate polishing process is carried out by using the abrasive member having the hardness higher than the abrasive member used for the rough polishing process, and then a final polishing process is carried out by using the abrasive member having the hardness higher than the abrasive member used for the intermediate polishing process.
In the optical fiber end face polishing method, since the end face of the ferrule is polished by the abrasive member which is fixed to the rotated or revolved polishing board, for example, by rough-polishing the end face of the ferrule in the rough polishing process, a peripheral portion of a radial outside in the end face of the ferrule is gradually rough-polished and the end face of the ferrule is formed to be a convex sphere.
After the rough polishing process, the intermediate polishing process and the final polishing process are carried out in order, and in these cases, the polishing is carried out in a manner that the abrasive member used in the immediately previous polishing process is changed for the abrasive member having the hardness higher than the abrasive member used in the immediately previous polishing process. That is to say, the end face of the ferrule is polished in a manner that the final polishing process is carried out with the abrasive member having the hardness higher than the abrasive member used for the intermediate polishing process after the intermediate polishing process is carried out with the abrasive member having the hardness higher than the abrasive member used for the rough polishing process. This allowed the mirror surface finish to be carried out in such a manner that, in a neighborhood of the end face of the optical fiber in the end face of the ferrule, the end face of the ferrule in the shape of the convex sphere is gradually polished to be smooth.
However, in the above-described optical fiber end face polishing apparatus, since the end face of the ferrule is polished under pressure to the abrasive member with a given load while the abrasive members are changed in the order from those of the lower hardness to those of the higher hardness in each of the polishing processes, in the intermediate and final polishing processes after a nearly central portion of the end face of the ferrule is formed to be the convex in the first rough polishing process, polishing pressure against the end face of the ferrule is gradually decreased in a radially outward direction. That is to say, in the intermediate and final polishing processes, when the end face of the ferrule is polished under pressure, a change in an elastic deformation of the abrasive member is decreased compared with the rough polishing process, an area where the end face of the ferrule abuts on the abrasive member is gradually decreased toward the direction of the end face of the optical fiber. Therefore, there is a problem that the polishing pressure applied to the neighborhood of the end face of the optical fiber in the end face of the ferrule is increased, the peripheral portion of the radial outside in the end face of the ferrule is not polished in the intermediate and final polishing processes, consequently, a polishing residue of the rough polishing process is generated.
For example, when the ferrule in which the polishing residue has been generated is oppositely connected to an optical connector adaptor, there is a problem that the peripheral portion of the end face of the ferrule contacts an inside surface of a sleeve, which causes the flaw to be generated in the inside surface of the sleeve.
Since the polishing pressure applied to the nearly central portion of the end face of the ferrule is increased with proceeding from the rough polishing process to the subsequent polishing processes, there are a problems that the neighborhood of or region near the end face of the optical fiber in the end face of the ferrule is concaved by being excessively polished or that a scratch is generated in the end face in case that some foreign matter enters between the end face of the ferrule and the polishing surface of the polishing member.
Furthermore, though abrasive particles which polish the end face of the ferrule intervene on the polishing surface of the polishing member, when the polishing pressure applied to the neighborhood of the end face of the optical fiber in the end face of the ferrule is large, a damaged layer is generated in the end face of the ferrule by an influence of the abrasive particle. This causes a problem that return loss is decreased when the ferrules are oppositely connected each other.